(1) Field of the Invention
This invention generally relates to acoustic imaging and more specifically to apparatus for imaging objects using acoustic energy, particularly objects located underwater.
(2) Description of the Prior Art
Two basic approaches are used to produce visual representations of objects in an underwater environment that are characterized by various degrees of spatial resolution. Conventional underwater photographic apparatus provides the best spatial resolution. However, this apparatus relies on reflected energy in the visible light spectrum and therefore often requires separate light sources in order to obtain an image. Personnel may be required to operate the apparatus. Light transmission underwater, particularly in the oceans, is quite limited, so the range of any such photographic apparatus can be limited.
When an object is located in the ocean, wave and wind action produce acoustic energy that travels great distances. As a result, background acoustic energy is nearly always present from a source. Hydrophone detectors for monitoring acoustic energy in water are well known and can detect such acoustical energy at significant distances from the effective source. Such hydrophones typically include a piezoelectric or similar transducer that converts the acoustic energy into a corresponding electrical signal. In a simple form, a single hydrophone or multiple hydrophones deployed as listening devices merely detect sounds that objects make. Such apparatus does not obtain sufficient statistical information for producing a visual representation of an object.
Two-dimensional arrays of hydrophones can yield sufficient statistical information of spatial resolution that enables the image to depict the profile of an object. That is, if an object is located intermediate the effective source and the array, the object will block some acoustic energy from reaching portions of the array and the image will appear negatively on a display. Conversely, if an acoustic generator produces an energy pulse, acoustic energy reflected from the object will appear positively on a display. However, any such array must include a large number of elements to obtain sufficient statistical information to produce an image with any reasonable level of spatial resolution. It is also necessary to provide some selectivity in accepting energy in order to obtain any degree of selectivity with respect to a field of view.
Improved spatial and field of view selectivity resolution can be obtained if each hydrophone in the array drives a beam forming circuit. Beam forming circuits are essentially variable time delay circuits that produce discrete beams. However, beam forming circuits are expensive and each direction requires an independent beam forming circuit. Consequently when a large number of hydrophones, as required to obtain reasonable spatial resolution are used, the cost of the array and beamformer can become prohibitive. For example, if it were desired to provide an array that would have a spatial resolution of about one foot (1/2 wavelength at 5000 Hz), the array would require a detector about every six inches. A 100 foot by 100 foot array would require over 40,000 hydrophones and consequently over 40,000 beam forming circuits.
Ultrasound imaging apparatus used for medical diagnostics provides visual images that have reasonable spatial resolution as discussed in the following patents:
U.S. Pat. No. 4,434,658 (1984) Miyazaki et al PA1 U.S. Pat. No. 4,511,998 (1985) Kanda et al PA1 U.S. Pat. No. 4,596,145 (1986) Smith et al PA1 U.S. Pat. No. 4,706,185 (1987) Karaki et al PA1 U.S. Pat. No. 4,908,774 (1990) Lund et al
In Miyazaki et al an ultrasonic energy source directs acoustic energy toward an object to be imaged. Acoustic energy passing then passes through an acoustic lens to be focused on a detector array. The detector array includes a piezoelectric transducer with a common electrode, a plurality of discrete planar electrodes spaced from the common electrode and piezoelectric material between the common electrode and the discrete electrodes. Each of the discrete electrodes connects through multiplexing and gating circuits to provide an input to an amplifier to produce imaging information for a visual display.
Kanda et al disclose a multi-element acoustic transducer for a scanning acoustic microscope. A first transducer generates a focused acoustic beam and a second transducer detects acoustic energy after it traverses an image. The acoustic transducers comprise an array of elements with a common electrode having an array of spherical portions formed on the surface thereof. A piezoelectric material coats this common electrode including the is spherical portions and individual spherical electrodes, organized according to the array attach to the other side of the piezoelectric material. The resulting laminated structure provides an array of piezoelectric transducers. A multiplexing network couples individual discrete electrodes to a single amplifier for subsequent processing to produce an image.
Smith et al disclose an acoustic imaging system for producing projection images of a three-dimensional volume by using transmitted acoustic pulses and a parallel signal processing apparatus. Signals generated by the transducers in response to received acoustic energy pass through a series of summing circuits, rectifying circuits, filter circuits, multiplier circuits and related circuits to be processed and produce the image.
Karaki et al disclose an apparatus for displaying ultrasonic images. An acoustic lens focuses the ultrasonic image on a detector array. Memories receive and store three ultrasonic images during each scan of an object thereby to enable the construction of a color image.
Lund et al disclose an ultrasonic system with a probe that moves over a surface of an object along a rectilinear scanning path. Signals received at each position are processed and stored in a corresponding memory position. Other circuitry utilizes the information in the memory to produce a visual display.
Although each of the foregoing patents discloses ultrasonic imaging apparatus for producing images of good spatial resolution in a medical environment, the technology in these patents is not readily adapted for underwater imaging applications. For example, ultrasonic imaging apparatus in medical diagnostics uses a very small field of view and is always proximate the object being imaged. It is not readily adapted for imaging large geographic areas with objects that are remote from the apparatus. Each of the disclosed apparatus is a "pulse-echo" type of apparatus in which an ultrasonic transducer generates an acoustical pulse and a detector array for receiving energy from the object. In many underwater imaging applications the generation of such a pulse is not desirable. Consequently apparatus including hydrophones, beam formers and other redundant and expensive circuitry has continued to be the apparatus of choice for underwater imaging even though it is complex, expensive and provides images of limited spatial resolution.